Herpes simplex virus (HSV) is a highly prevalent neurotropic virus. Although on the whole, chronic, latent or persistent infection is considered to be relatively benign, HSV infections can cause cognitive impairment during and after acute encephalitis. Some studies have documented cognitive impairment in exposed persons that is untraceable to encephalitis. Most studies have focused on these impairments in the mentally ill, mostly among individuals with schizophrenia, and only recently have studies begun to examine the impact of HSV infection on the cognition of healthy individuals.

Subjects were a representative, random sample of 612 soldiers before active duty in the Israeli military (Israeli defense force — IDF), 62.2% HSV positive (n = 381) and 38.8% HSV negative (n = 231). Cognitive functioning and language abilities were compared between these groups, controlling for years of education, immigration status, and gender.

Compared to soldiers who were sero-negative, soldiers who were sero-positive for HSV had significantly lower IQ scores (IQ = 97.96, SD = 15.19 vs IQ = 103.23, SD = 14.23; p ≤ 0.001, effect size (ES) = 0.2), and significantly lower Hebrew language scores (ES = 0.1, p ≤ 0.01). The results remained significant after removing subjects with mild depression, anxiety or personality disorders. Although we could not control for socio-economic status directly, our findings indicate that infection with HSV-1 is associated with reduced cognitive functioning in healthy individuals. This finding adds to the growing number of studies in the schizophrenia literature and indicates that many research findings seemingly characteristic of schizophrenia are related to the association between HSV exposure and cognitive functioning in general, and are not illness specific.

(interesting that similar was found in another study recently wrt previous exposure to another virus)

Not much use until they find a way to control for those socioeconomic factors. And the correlation might suggest causation in the opposite direction of what they are proposing - people with lower IQs are more likely to engage in behavior (unsafe sex), which results in higher rates of HSV.

The bottom of this table (from this study) shows the prevalence of HSV-1 for different education levels.

I wonder if APOE gene variants play a role in the degree of cognitive impairments / IQ loss linked to HSV-1? The APOE-4 form of the APOE gene apparently allows HSV-1 to enter the brain, and HSV-1 has been linked to Alzheimer's disease in those who possess the APOE-4 variant of APOE. 1

Cognitive impairments have also been found in those with bipolar disorder and HSV-1, 1 and those with schizophrenia and HSV-1. 1

Mild cognitive impairment (MCI) subjects carrying the APOE epsilon4 allele showed distinct cognitive and imaging profiles, which appeared to resemble those of early Alzheimer patients. APOE epsilon4 genotype was associated with greater impairments in memory and functional activities as well as hippocampal atrophy.

It would be interesting to know whether individuals with the APOE-4 variant and found to have increased cognitive impairment in these studies also carried the herpes simplex 1 virus.

Might it be that you only get increased cognitive impairment from the APOE-4 variant if you carry HSV-1? Remember that the APOE-4 variant allows HSV-1 to enter the brain (APOE-4 facilitates HSV-1 latency in the brain 1).

For those who have done the 23andme test, you can see whether you have the APOE-4 variant (and thus a greater risk forAlzheimer's) on this page.

Not much use until they find a way to control for those socioeconomic factors. And the correlation might suggest causation in the opposite direction of what they are proposing - people with lower IQs are more likely to engage in behavior (unsafe sex), which results in higher rates of HSV.

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Or just more kissing.

This study is a classical example of why correlation should not be assumed to be causation.

I haven't accessed the full article but it seems they also haven't controlled for drug use, in particular marijuana use. In Israel it's quite prevalent and while illegal not at all enforced. Marijuana use will definitely bring down those cognitive scores.

I haven't accessed the full article but it seems they also haven't controlled for drug use, in particular marijuana use. In Israel it's quite prevalent and while illegal not at all enforced. Marijuana use will definitely bring down those cognitive scores.

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Why would they have to control for marijuana (or any other intoxicant) use? I don't think there is any special propensity for those with herpes simplex to use marijuana.

Why would they have to control for marijuana (or any other intoxicant) use? I don't think there is any special propensity for those with herpes simplex to use marijuana.

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Sorry I thought I read somewhere that drug use could cause HSV-1 to have increased neurotropism, so it would be necessary to control for this since it's possible HSV-1+ people who didn't use drugs wouldn't have (as much) virus in their CNS.

According to a study in Ontario, up to 55% of Canadians age of 15 to 16, and 89% of individuals in their early forties have antibodies to HSV-1

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I think HSV2 is much less common and is highly correlated with socio-economic status. I think that is much less true of HSV1. This may be because HSV1 typically causes oral lesions from which the virus can easily spread without risky behavior, while HSV2 typically causes genital lesions from which viral transmission requires more, erm, intimate exposure. HSV1 is commonly acquired during childhood when risky behaviors such as unsafe sex are unlikely.

Worldwide rates of either HSV-1 or HSV-2 are between 60% and 95% in adults.[9] HSV-1 is usually acquired during childhood.[1] Rates of both increase as people age.[9] Rates of HSV-1 are between 70% and 80% in populations of low socioeconomic status and 40% to 60% in populations of improved socioeconomic status.[9]

Sorry I thought I read somewhere that drug use could cause HSV-1 to have increased neurotropism, so it would be necessary to control for this since it's possible HSV-1+ people who didn't use drugs wouldn't have (as much) virus in their CNS.

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Ah right.

I found a few studies like this one that noted a decreased immune resistance to HSV-2 vaginal infection in the mouse on exposure to THC from Cannabis. Though this study found the opposite: that THC has an antiviral effect against both HSV-1 and HSV-2. This article also talks about the antiviral effects of THC.

So it does not seem clear whether THC inhibits or promotes herpes simplex virus.